Electrical devices such as, for example, soft starters, overload protection relays or motor starters are employed in the industrial automation field for the purpose of controlling or monitoring a three-phase alternating-current (AC) motor.
In addition to 3-phase controlled soft starters (three-phase power controllers), 2-phase controlled soft starters are also used in electrical soft starter devices for three-phase AC motors. In the case of the 3-phase controlled soft starters, three current transformers are present in most cases for the purpose of controlling and measuring the current in the three conductors (phases) of the soft starter device. In 2-phase controlled soft starters, one of the three conductors (phases) of the soft starter device is bridged and in principle one current transformer can be dispensed with (in the bridged conductor) in order to perform the soft start of the motor. The energy supply for the three-phase AC motor connected to the soft starter device is conducted and controlled by way of the three conductors of the soft starter device so that for example a defined startup behavior of the three-phase AC motor can be achieved.
The current measurement signals of the current transformers of the soft starter device can also be used inter alia for detecting a phase failure. However, it is problematic in this case if no current transformer is present in a conductor (phase) of the current transformer. This means that a phase failure in the conductor in which no current transformer is present cannot be detected and the motor would continue running on two phases until it was switched off by another protection mechanism. What is present in particular in the case of a phase failure is an interruption of a current that is to be transmitted inside the conductor.
A phase failure can also be detected with the aid of an analog voltage measurement. The absence of a voltage can be detected relatively easily if the voltage in a conductor falls below a specified threshold value. However, the corresponding hardware and firmware are necessary for this purpose so that said detection can be realized.
Phase failure detection on the basis of the current has hitherto been accomplished by way of three current transformers. If a current in a conductor falls below a specified threshold value, it can be assumed that no voltage is present at the conductor.